Around a building perimeter, the gap between partition walls and window mullions or partition walls and glass curtainwalls can present aesthetic, sound, and material choice and installation quandaries. Mullion Mate and Window Mate from Gordon Inc. are adjustable two-piece extruded aluminum gap closure elements used to block the space and create a transition between window mullions or windows and partitions. Gordon recently performed a series of independent lab tests, using ASTM E90-09 (2016), with distinct setups where the gap closures manufactured by Gordon were tested on walls with different STCs and configurations mimicking true field installation boundaries. Testing showed the weakest barrier of sound attenuation—in this case, the window mullion (tube), when present—dictated the resulting composite STC. Additionally, for the wall and gap closures tested, the STC of the wall dictates the high limit of the composite STC.